[PATCH RFC 04/29] mm/page_alloc: allow for making page types sticky until freed

[Harry Yoo]

On Mon, Jun 23, 2025 at 05:26:05PM +0200, David Hildenbrand wrote:
> On 18.06.25 20:08, Zi Yan wrote:
> > On 18 Jun 2025, at 14:04, Zi Yan wrote:
> > 
> > > On 18 Jun 2025, at 13:39, David Hildenbrand wrote:
> > > 
> > > > Let's allow for not clearing a page type before freeing a page to the
> > > > buddy.
> > > > 
> > > > We'll focus on having a type set on the first page of a larger
> > > > allocation only.
> > > > 
> > > > With this change, we can reliably identify typed folios even though
> > > > they might be in the process of getting freed, which will come in handy
> > > > in migration code (at least in the transition phase).
> > > > 
> > > > Signed-off-by: David Hildenbrand <david at redhat.com>
> > > > ---
> > > >   mm/page_alloc.c | 3 +++
> > > >   1 file changed, 3 insertions(+)
> > > > 
> > > > diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> > > > index 858bc17653af9..44e56d31cfeb1 100644
> > > > --- a/mm/page_alloc.c
> > > > +++ b/mm/page_alloc.c
> > > > @@ -1380,6 +1380,9 @@ __always_inline bool free_pages_prepare(struct page *page,
> > > >   			mod_mthp_stat(order, MTHP_STAT_NR_ANON, -1);
> > > >   		page->mapping = NULL;
> > > >   	}
> > > > +	if (unlikely(page_has_type(page)))
> > > > +		page->page_type = UINT_MAX;
> > > > +
> > > >   	if (is_check_pages_enabled()) {
> > > >   		if (free_page_is_bad(page))
> > > >   			bad++;
> > > > -- 
> > > > 2.49.0
> > > 
> > > How does this preserve page type? Isn’t page->page_type = UINT_MAX clearing
> > > page_type?
> > 
> > OK, next patch explains it. free_pages_prepare() clears page_type,
> > so that caller does not need to.
> > 
> > I think the message is better to be
> > 
> > mm/page_alloc: clear page_type at page free time
> > 
> > page_type is no longer needed to be cleared before a page is freed, as
> > page free code does that.
> > 
> > With this change, we can reliably identify typed folios even though
> > they might be in the process of getting freed, which will come in handy
> > in migration code (at least in the transition phase).
> 
> 
> I'll change it to
> 
>     mm/page_alloc: let page freeing clear any set page type
>     Currently, any user of page types must clear that type before freeing
>     a page back to the buddy, otherwise we'll run into mapcount related
>     sanity checks (because the page type currently overlays the page
>     mapcount).
>     Let's allow for not clearing the page type by page type users by letting
>     the buddy handle it instead.
>     We'll focus on having a page type set on the first page of a larger
>     allocation only.
>     With this change, we can reliably identify typed folios even though
>     they might be in the process of getting freed, which will come in handy
>     in migration code (at least in the transition phase).

Acked-by: Harry Yoo <harry.yoo at oracle.com>

-- 
Cheers,
Harry / Hyeonggon